Conventionally, clock generators used in digital audio equipment in the acoustic art generally rely on the use of a quartz oscillator in performing analog-to-digital conversion or digital-to-digital conversion to record digital data and/or digital-to-analog conversion or digital amplification to reproduce digital data.
One example of such conventional clock generators is shown in FIG. 3 hereof. The conventional clock generator is comprised of a reference quartz oscillator 8 built-in or modularly included in a body 7 of digital audio equipment for generating a clock signal required for operation of the digital audio equipment.
FIG. 4 shows another example of conventional clock generator 9 and a body 10 of digital audio equipment that are used in combination to form a digital audio system. Though not shown, the clock generator 9 has a built-in reference quartz oscillator or is constructed to accept the input of a reference signal from an external reference quartz oscillator, so that a clock signal required for operation of the digital audio equipment is generated by the clock generator 9 and supplied to the digital audio equipment body 10.
However, due to low frequency stability (or uncertainty) of the quartz oscillator, the conventional use of such quartz oscillators as reference oscillators raises a problem that sound, processed either in the form of digital or analog, involves very small fluctuations, which will lower the sound quality and eventually provide physiological discomfort and unnaturalness to human users. Through researches the present inventors have found that the sound quality is improved as the accuracy of an oscillator used increases. Based on this founding, an atomic clock (also called “atomic frequency standard”), which uses atoms of cesium, hydrogen or mercury and whose accuracy or uncertainty is remarkably higher than that of the conventional quartz oscillators, is used according to the present invention for the purpose of improving the sound quality in proportion to the frequency stability (uncertainty) of the atomic clock. Stated in other words, since the frequency stability of the conventional quartz oscillators is insufficient to perform analog-to-digital conversion, digital-to-digital conversion and digital-to-analog conversion, such conversion processes when effected necessarily involve generation of great jitter, failing to provide recorded or reproduced sounds with high sound quality and high fidelity. This may result in unsatisfactorily matched reality and presence.
Frequency stability (uncertainty) of the conventional quartz oscillators is normally of the order of from 1 to 100 parts in 10 to 4th (i.e., 0.0001 to 0.000001). On the other hand, the resolution of analog-to-digital converters, digital-to-digital converters or the digital-to-analog converters that can be used in digital audio equipment is usually expressed in bits. In case of a converter having a resolution of 16 bits, the least significant increment is 1/65,536 since 2.sup.16=65,536. This means that the 16-bit converter requires frequency stability of less than 1/65,536 so as to meet a necessary condition and of less than 1/66,000,000 so as to meet a necessary and sufficient condition. In case of a converter having a resolution of 20 bits, the least significant increment is 1/1,048,576, which will require frequency stability of less than 1/10,000,000 to meet a necessary condition and of less than 1/10,000,000 to meet a necessary and sufficient condition. Similarly, in case of a converter having a resolution of 24 bits, the least significant increment is 1/16,777,216, which will require frequency stability of less than 1/170,000,000 to meet a necessary condition and of less than 1/170,000,000 to meet a necessary and sufficient condition.
It appears clear from the foregoing discussion that the frequency stability of the conventional quarts oscillators does not follow or support the resolutions of analog-to-digital converters, digital-to-digital converters and digital-to-analog converters used in the digital audio equipment. Accordingly, jitter of a digital signal produced after analog-to-digital conversion and a jitter caused in the digital-to-digital converter or the digital-to-analog converter may produce a noise, which will deteriorate sound resolution and thus hinder recording and/or reproduction of sound with high sound quality and high fidelity. The sound is rendered unnatural and, hence, satisfactorily matched reality and presence cannot be provided.